Some comparative aspects of regulation of muscle and liver phosphorylase kinase

The interaction of muscle and liver phosphorylase kinase with some proteins has been studied. It was shown that muscle G-actin has a visible stimulating effect on the dephosphorylated form of muscle phosphorylase kinase. The effect of F-actin on this enzyme is very low. The interaction of phosphorylase kinase with G-actin probably is one of the additional links between glycogenolysis and muscle contraction. To answer the question what subunit(s) of phosphorylase kinase is involved in the interaction with G-actin we studied the influence of actin on the kinase preparations previously activated to a different degree by partial proteolysis with endogenous protease(s) or with trypsin. G-actin has almost no stimulating effect on the preparations of phosphorylase kinase deeply activated by proteolysis (pH 6.8-8.2 activity ratio more than 0.2). The experiments with partial proteolysis allow us to suppose that alpha-subunit is involved in the interaction of phosphorylase kinase with G-actin. Skeletal muscle G-actin activates purified preparations of liver phosphorylase kinase but to a lower degree than muscle enzyme. Brain and liver calmodulin has a low activating effect on liver phosphorylase kinase in the presence of calcium. Calcium-independent action of calmodulin on the preparations of liver phosphorylase kinase is stronger; probably it is connected with a nonspecific effect of this small acidic protein on the liver enzyme. The basic protein protamine has a strong inhibitory effect on liver phosphorylase kinase.